1. Field of the Invention
The present invention relates to a production process of a Mnxe2x80x94Zn ferrite, and more particularly to a production process of a Mnxe2x80x94Zn ferrite that enables wastes of sintered products to be recycled.
2. Description of the Related Art
Typical oxide magnetic materials having soft magnetism include a Mnxe2x80x94Zn ferrite that has been used as a low loss material used for switching power transformers, flyback transformers and the like, various inductance elements, an impedance element for EMI countermeasures, an electromagnetic wave absorber and the like. Conventionally, this Mnxe2x80x94Zn ferrite usually has a basic component composition containing over 50 mol % (52 to 55 mol % on the average) Fe2O3, 10 to 24 mol % ZnO and the reminder MnO. The Mnxe2x80x94Zn ferrite is usually produced by mixing respective material powders of Fe2O3, ZnO and MnO in a prescribed ratio, subjecting mixed powders to the respective steps of calcination, milling, component adjustment, granulation and pressing to obtain a desired shape, then performing sintering treatment at 1200 to 1400xc2x0 C. for 3 to 4 hours in a reducing atmosphere in which a relative partial pressure of oxygen is considerably lowered by supplying nitrogen. The reason why the Mnxe2x80x94Zn ferrite is sintered in the reducing atmosphere is that when the Mnxe2x80x94Zn ferrite containing over 50 mol % Fe2O3 is sintered in the air, densification is not attained sufficiently thereby failing to obtain excellent soft magnetism, and that Fe2+ which has positive crystal magnetic anisotropy is formed by reducing a part of Fe2O3exceeding 50 mol % thereby canceling negative crystal magnetic anisotropy of Fe3+ and enhancing soft magnetism.
Amount of the above-mentioned Fe2+ formed depends on relative partial pressures of oxygen in sintering and cooling after the sintering. Therefore, when the relative partial pressure of oxygen is improperly set, it becomes difficult to ensure excellent soft magnetic properties. Thus, conventionally, the following expression (1) has been experimentally established and the relative partial pressure of oxygen in sintering and in cooling after the sintering has been controlled strictly in accordance with this expression (1).
xe2x80x83log Po2=xe2x88x9214540/(T+278)+bxe2x80x83xe2x80x83(1)
where T is temperature (xc2x0 C.), Po2 is a relative partial pressure of oxygen, and b is a constant, which is usually 7 to 8.
In addition, the above-mentioned milling step is conducted so that an average grain size of a fine milled powder ranges 1.0 to 1.4 xcexcm. If the average grain size is more than 1.4 xcexcm, a desired density can not be obtained in sintering, and if the average grain size is less than 1.0 xcexcm, it becomes difficult to handle the resultant powder.
A large amount of wastes are generated for several reasons, such as a surplus, defects and the like in each step of the production process of Mnxe2x80x94Zn ferrite described above. While wastes generated prior to the step of pressing can be recycled without particular problems, wastes generated in the step of sintering due to defects, such as dimensional error, cracking, chipping or the like, are difficult to recycle for the reason described below and are just scrapped as they are.
The step of sintering a Mnxe2x80x94Zn ferrite is largely affected by vacancy concentration of oxygen ions that have the lowest diffusing rate along its constituent ions. As the vacancy concentration of oxygen ions increases, the diffusion of oxygen ions, iron ions, manganese ions and zinc ions is accelerated and the sintered density increases. Fe2O3 content and a relative partial pressure of oxygen in an atmosphere are factors governing the vacancy concentration of oxygen ions. The smaller the Fe2O3 content is and the lower the relative partial pressure of oxygen is, the easier the vacancies of oxygen ions can be formed. Because a conventional Mnxe2x80x94Zn ferrite contains over 50 mol % Fe2O3, the vacancies of oxygen ions decrease, whereas the respective vacancies of iron ions, manganese ions and zinc ions increase. That is, in case a conventional sintered Mnxe2x80x94Zn ferrite is milled and pressed for recycling, it must be sintered with the relative partial pressure of oxygen in an atmosphere considerably lowered. However, the lowest relative partial pressure of oxygen available in actual mass production process is about 0.0001 in which a desired vacancy concentration of oxygen can not be obtained. As a result of this, the sintering can not be conducted smoothly making it difficult to obtain a desired density. Consequently, the resultant sintered cores do not have magnetic properties good enough to serve for practical use and therefore are simply scrapped.
The present invention has been made in consideration of the above-mentioned conventional problems, and an object of the present invention is therefore to provide a production process of a Mnxe2x80x94Zn ferrite, which enables wastes of sintered cores to be recycled without serious difficulties in sintering.
In order to attain the above-mentioned object, a production process of a Mnxe2x80x94Zn ferrite according to the present invention comprises the steps of; milling a sintered core of Mnxe2x80x94Zn ferrite for recycling; subjecting a recycled powder to a component adjustment so as to have a composition of 44.0 to 49.8 mol % Fe2O3, 4.0 to 26.5 mol % ZnO, 1.0 to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn2O3 and a remainder being MnO; pressing a mixed powder subjected to the component adjustment; and sintering a green compact obtained by pressing the mixed powder.
Amount of powder to be recycled, that is a recycled powder, can be arbitrarily selected. When the recycled powder has a target component composition, all mixed powder for pressing may be recycled. And, when the recycled powder does not have a target component composition, the components must be adjusted by appropriately adding respective raw material powders of Fe2O3, ZnO, CoO, MnO or the like.
As Fe2O3 content is restricted to less than 50 mol % in the present invention as mentioned above, vacancies of oxygen ions in the sintering step and the density of a sintered core is easily increased. Therefore, when the sintering (heatingxe2x80x94maintaining temperaturexe2x80x94cooling) is conducted in an atmosphere containing an appropriate amount of oxygen, the resultant sintered core has sufficiently high density even if a recycled powder is used. However, as too small Fe2O3 content results in lowering the initial permeability, at least 44.0 mol % Fe2O3 must be contained in the ferrite.
Also, as Fe2O3 content is restricted to less than 50 mol % in the present invention as mentioned above, Fe2+ is little formed. Since Co2+ in CoO has a positive crystal magnetic anisotropy, CoO can cancel out a negative crystal magnetic anisotropy of Fe3+ even if Fe2+ having a positive crystal magnetic anisotropy does not exist. However, when CoO content is too small, the effect is small. On the contrary, when the CoO content is too large, the magnetic strain increases and the initial permeability is lowered. Thus, the CoO content is set to 0.1 to 3.0 mol %.
ZnO influences the Curie temperature and saturation magnetization. Too large amount of ZnO lowers the Curie temperature to result in practical problems, but on the contrary, too small amount of ZnO reduces the saturation magnetization, so it is desirable for ZnO content to be set to the above-mentioned range of 4.0 to 26.5 mol %.
A manganese component in the above-mentioned ferrite exists as Mn2+ and Mn3+. Since Mn3+ distorts a crystal lattice, thereby significantly lowering the initial permeability, Mn2O3 content is set to 1.00 mol % or less. However, since too small Mn2O3 content lowers the electrical resistivity significantly, at least 0.02 mol % Mn2O3 must be contained in the ferrite.
It is desirable for the lower limit of the average grain size of the recycled powders to be set to about 1.0 xcexcm similarly to the prior art. However, even when the average grain size exceeds 1.4 xcexcm and measures, for example about 2.0 xcexcm, sufficiently high density is obtained in the sintering.
Since the present invention relates to recycling wastes of sintered cores, a recycled ferrite naturally contains additives contained in the wastes of sintered cores. Generally, CaO, SiO2, ZrO2, Ta2O5, HfO2, Nb2O5, V2O5, Bi2O3, In2O3, Cuo, MoO3, WO3, Al2O3, TiO2 and SnO2 are used as additive. Therefore, the recycled ferrite in the present invention may contain a slight amount of one or more of these additives.
In the present invention, the above-mentioned Mn2O3 content can be controlled by sintering in an atmosphere of an adjusted relative partial pressure of oxygen. In this case, it is desirable to control the Mn2O3 content, that is the amount of Mn3+, by sintering and cooling after the sintering in an atmosphere of a relative partial pressure of oxygen obtained by using an arbitrary value in a range of 6 to 10 as the constant b in the expression (1). When a value larger than 10 is selected as the constant b, the amount of Mn3+ in the ferrite exceeds 1 mol %, whereby the initial permeability is rapidly lowered. Therefore, the amount of Mn3+ in the ferrite must be reduced to increase the initial permeability. Thus, it is desired that a small value be selected as the constant b. However, when a value smaller than 6 is selected, Fe2+ increases or Mn3+ decreases too much, thereby significantly lowering the electrical resistivity. Accordingly, the constant b is set to 6 at smallest. A relative partial pressure of oxygen (Po2) may be set to a range of 0.0001 to 0.21, where the upper limit of 0.21 corresponds to the atmospheric pressure, and the lower limit of 0.0001 can be obtained in actual production process without serious difficulty.
In the present invention, Fe2O3 content is restricted to less than 50 mol % and a constant b in the expression (1) is set to an arbitrary value selected from a range of 6 to 10 as mentioned above. Therefore, the electrical resistivity of the resultant Mnxe2x80x94Zn ferrite is 10 xcexa9m or more that is much higher than that of the conventional Mnxe2x80x94Zn (about 0.01 to 1 xcexa9m). Thus, for example, the Mnxe2x80x94Zn ferrite of the present invention is suitable for a magnetic material used in a high frequency region exceeding 1 MHz.
In production of the Mnxe2x80x94Zn ferrite, wastes of sintered Mnxe2x80x94Zn ferrite generated in sintering step are milled with appropriate milling measures, for example a hammer mill and a jet mill to obtain a recycled powder, and respective raw material powders of Fe2O3, ZnO, CoO, MnO and the like as main components are mixed with the recycled powder in a prescribed ratio to obtain a mixed powder having a target component composition. The recycled powder does not have to be grained at the beginning and may have an average grain size of about 40 xcexcm or less. In this case, the mixed powder described above is calcined, then finely milled to an average grain size of about 2 xcexcm or less. The temperature for the calcination can be appropriately selected from a range of 850 to 950xc2x0 C. depending on a target composition. However, if the amount of the raw material powders to be added to the recycled powder is slight, the calcination can be omitted. Further, a general-purpose ball mill can be used for the fine milling of the calcined powder. Then, the respective powders of several additives described above are mixed as required with the fine mixed powders in a prescribed ratio to obtain a mixture having a target component composition. Subsequently, the mixture is granulated and pressed in accordance with a usual ferrite production process, and then sintered at 1200 to 1400xc2x0 C. for 2 to 4 hours.
In the above-mentioned sintering and cooling after the sintering, a relative partial pressure of oxygen is controlled by flowing inert gas such as nitrogen gas or the like into a sintering furnace. In this case, the constant b in the expression (1) can be arbitrarily set to a value selected from a range of 6 to 10. Further, in this case, since the reaction of oxidation or reduction can be neglected independent of relative partial pressures of oxygen at a temperature of below 500xc2x0 C., the cooling after the sintering is to be conducted in accordance with the above-mentioned expression (1) only till the temperature gets down to 500xc2x0 C.